binder: Parcel avoid too small initial allocation

When a transaction is being sent, frequently many allocations are in a
row. In order to avoid all these allocations, creating a minimum
allocation size here. A test is also added which ensures that a basic
transaction takes exactly one allocation.

Bug: 150149045
Test: atest binderAllocationLimits
Change-Id: I88ab066328158a8b73dd81201b796f247c4dbbd0