freezer: reentrant enable/disable method

the app freezer state can be toggled in multiple situations when a
debugging tool like heapdump is invoked, or when system_server dumps its
binder connections. All these uses are potentially concurrent and a
robust method to handle reentrancy is necessary to avoid leaving the
freezer in a state incompatible with a specific operation.

This patch moves freezer enable and disable operations from Process to
CachedAppOptimizer (ActivityManager), introduces a new ActivityManager API
to centralize all freezer state changes to ActivityManager and modifies
client code accordingly.

Bug: 151225245
Test: manually verified that no regression are introduced on freezer
behavior, verified that concurrent behavior is handled properly

Change-Id: I7d588cc6e0499012dce64ed4e42ff2adb336062d
Merged-In: I7d588cc6e0499012dce64ed4e42ff2adb336062d