Added|Changed!: Implement new design for shell environment generation and add support for MIT licensed shell environment client

- `ShellEnvironmentClient` has been renamed to `IShellEnvironment` with certain changes to its interface methods, including requirement for `Execution` command itself for `setupShellCommandEnvironment()`.
- `UnixShellEnvironment` implements the `IShellEnvironment` interface as is the abstract base class of all other shell environments.
- `AndroidShellEnvironment` extends from the `UnixShellEnvironment` class and provides an environment that would work for Android shells. This is `MIT` licensed and can be used by users importing the `termux-shared` library or the library itself to run `AppShell` shells. Previously, `TermuxShellEnvironmentClient` existed which was `GPLv3` licensed and it would not have been possible to use it for non-GPL code.
- `TermuxShellEnvironment` extends from the `AndroidShellEnvironment` class and adds/overrides additional environment variables required for Termux shells to work, including setting `HOME`, `TMPDIR`, `PATH` and `LD_LIBRARY_PATH` appropriately. Termux app related variables will be added in a later commit. `TermuxShellEnvironment` replaces `TermuxShellEnvironmentClient` and is `GPLv3` licensed.

app/src/main/java/com/termux/app/TermuxService.java

@@ -28,7 +28,7 @@ import com.termux.shared.errors.Errno;
 import com.termux.shared.shell.ShellUtils;
 import com.termux.shared.shell.command.runner.app.AppShell;
 import com.termux.shared.termux.settings.properties.TermuxAppSharedProperties;
-import com.termux.shared.termux.shell.TermuxShellEnvironmentClient;
+import com.termux.shared.termux.shell.command.environment.TermuxShellEnvironment;
 import com.termux.shared.termux.shell.TermuxShellUtils;
 import com.termux.shared.termux.TermuxConstants;
 import com.termux.shared.termux.TermuxConstants.TERMUX_APP.TERMUX_ACTIVITY;
@@ -469,7 +469,8 @@ public final class TermuxService extends Service implements AppShell.AppShellCli
         if (Logger.getLogLevel() >= Logger.LOG_LEVEL_VERBOSE)
             Logger.logVerboseExtended(LOG_TAG, executionCommand.toString());
 
-        AppShell newTermuxTask = AppShell.execute(this, executionCommand, this, new TermuxShellEnvironmentClient(), false);
+        AppShell newTermuxTask = AppShell.execute(this, executionCommand, this,
+            new TermuxShellEnvironment(),false);
         if (newTermuxTask == null) {
             Logger.logError(LOG_TAG, "Failed to execute new TermuxTask command for:\n" + executionCommand.getCommandIdAndLabelLogString());
             // If the execution command was started for a plugin, then process the error
@@ -578,7 +579,8 @@ public final class TermuxService extends Service implements AppShell.AppShellCli
         // Otherwise if command was manually started by the user like by adding a new terminal session,
         // then no need to set stdout
         executionCommand.terminalTranscriptRows = mProperties.getTerminalTranscriptRows();
-        TermuxSession newTermuxSession = TermuxSession.execute(this, executionCommand, getTermuxTerminalSessionClient(), this, new TermuxShellEnvironmentClient(), executionCommand.isPluginExecutionCommand);
+        TermuxSession newTermuxSession = TermuxSession.execute(this, executionCommand, getTermuxTerminalSessionClient(),
+            this, new TermuxShellEnvironment(), executionCommand.isPluginExecutionCommand);
         if (newTermuxSession == null) {
             Logger.logError(LOG_TAG, "Failed to execute new TermuxSession command for:\n" + executionCommand.getCommandIdAndLabelLogString());
             // If the execution command was started for a plugin, then process the error