Make TermuxTask and TermuxSession agnostic to termux environment

Those classes shouldn't be tied to termux environment like variables, interpreters and working directory since commands may need to be executed with a different environment like android's or with a different logic. Now both classes use the ShellEnvironmentClient interface to dynamically get the environment to be used which currently for Termux's case is implemented by TermuxShellEnvironmentClient which is just a wrapper for TermuxShellUtils since later implements static functions.

termux-shared/src/main/java/com/termux/shared/shell/ShellEnvironmentClient.java

@@ -0,0 +1,47 @@
+package com.termux.shared.shell;
+
+import android.content.Context;
+
+import androidx.annotation.NonNull;
+
+public interface ShellEnvironmentClient {
+
+    /**
+     * Get the default working directory path for the environment in case the path that was passed
+     * was {@code null} or empty.
+     *
+     * @return Should return the default working directory path.
+     */
+    @NonNull
+    String getDefaultWorkingDirectoryPath();
+
+    /**
+     * Get the default "/bin" path, likely $PREFIX/bin.
+     *
+     * @return Should return the "/bin" path.
+     */
+    @NonNull
+    String getDefaultBinPath();
+
+    /**
+     * Build the shell environment to be used for commands.
+     *
+     * @param currentPackageContext The {@link Context} for the current package.
+     * @param isFailSafe If running a failsafe session.
+     * @param workingDirectory The working directory for the environment.
+     * @return Should return the build environment.
+     */
+    @NonNull
+    String[] buildEnvironment(Context currentPackageContext, boolean isFailSafe, String workingDirectory);
+
+    /**
+     * Setup process arguments for the file to execute, like interpreter, etc.
+     *
+     * @param fileToExecute The file to execute.
+     * @param arguments     The arguments to pass to the executable.
+     * @return Should return the final process arguments.
+     */
+    @NonNull
+    String[] setupProcessArgs(@NonNull String fileToExecute, String[] arguments);
+
+}