Prepare for testing mainline module caches

The existing setTestMode(boolean) API is repurposed so that when the
test mode is enabled, all existing caches are put in test mode.  When
test mode is disabled, all existing caches are taken out of test mode.
(The "disabled" behavior is pre-existing.)  Also, any new cache is
created in the current test mode.

This behavior is consistent with the existing use case that is not
part of PIC unit tests.

The intent is to expose setTestMode as a mainline module API.

This makes setTestMode() a viable alternative to disableForTestMode(),
which is widely used but is sticky in a process.

Flag: android.app.pic_test_mode
Bug: 396173886
Test: atest
 * FrameworksCoreTests:PropertyInvalidatedCacheTests
 * FrameworksCoreTests:IpcDataCacheTest
 * CtsOsTestCases:IpcDataCacheTest
Change-Id: I1ad00f62d408b7261137bbd6bc47c394f0ee348f