ANDROID: Define/use MIGRATE_FALLBACKS

MIGRATE_PCPTYPES is used for two different purposes - one to specify
number of migrate types with PCP lists (as its name implies) and
the other as number of migrate-types that have fallbacks.

Rather than overload MIGRATE_PCPTYPES, define and use a second value,
MIGRATE_FALLBACKS to keep track of number of fallbacks. This would
be useful if we add items to one list but not the other (a follow on
patch will add MIGRATE_CMA to the pcp list but not to the fallbacks).

Bug: 330201547
Change-Id: Ife0af3a66f8d4fd12ca3642eea606076da3f7046
Suggested-by: Chris Goldsworthy <quic_cgoldswo@quicinc.com>
Signed-off-by: Sukadev Bhattiprolu <quic_sukadev@quicinc.com>