Use allocatable space when measuring for install.

The system is often willing to clear cached data to make room for
incoming installs, so use StorageManager.getAllocatableBytes() when
making "does it fit?" style decisions.

Add new INSTALL_ALLOCATE_AGGRESSIVE flag, which will flow through
to use StorageManager.FLAG_ALLOCATE_AGGRESSIVE when making allocation
related requests.  (This can be used by installers to indicate
packages that are critical to system health or security.

Test: runtest -x frameworks/base/core/tests/coretests/src/android/content/pm/PackageHelperTests.java
Bug: 36131437
Change-Id: If8118762fd1ca1f497d2cdd1787bdb3c9759dcc0